Processes and compositions for the reduction of nitrogen oxides in an effluent from the combustion of a carbonaceous fuel have been developed extensively over recent years. With the increased attention to the health risks and environmental damage caused by agents such as smog and acid rain, it is expected that NO.sub.x reduction research will continue to be pursued.
In an early application of the use of nitrogenous treatment agents to reduce NO.sub.x, Lyon in U.S. Pat. No. 3,900,554, describes a process for reducing nitrogen monoxide (NO) from combustion effluents by introducing ammonia or certain "ammonia precursors" into the effluent at temperatures which range from 1300.degree. F. to 2000.degree. F. In U.S. Pat. No. 4,208,386, Arand, Muzio, and Sotter improve on the Lyon process by teaching the introduction of urea for NO.sub.x reduction in oxygen-rich effluents at temperatures in the range of 1600.degree. F. to 2000.degree. F., when urea is introduced into the effluent alone, and 1300.degree. F. to 1600.degree. F. when urea is introduced with an ancillary reducing material. Arand, with Muzio and Teixeria, in U.S. Pat. No. 4,325,924 also teach the introduction of urea into fuel-rich combustion effluents to reduce NO.sub.x at temperatures in excess of about 1900.degree. F.
More recently, in a unique application of NO.sub.x reducing principles, Epperly, Peter-Hoblyn, Shulof, Jr., and Sullivan, in U.S. Pat. No. 4,777,024 disclose a method for achieving substantial nitrogen oxides reductions while minimizing the production of so-called secondary pollutants, such as ammonia and carbon monoxide, through a multiple stage injection process. Moreover, Epperly, O'Leary, and Sullivan, in U.S. Pat. No. 4,780,289 have disclosed a complementary process for achieving significant, and potentially maximized, NO.sub.x reductions while minimizing the production of secondary pollutants by utilizing the nitrogen oxides reduction versus effluent temperature curve of the treatment regimen being effected at each NO.sub.x reduction introduction in a combustion system. These inventions, though, focus mainly on minimizing the levels of ammonia and carbon monoxide while performing nitrogen oxides reductions and do not specifically address the generation of nitrous oxide.
Schell, in U.S. Pat. Nos. 4,087,513 and 4,168,299, discloses processes for the hydrolyzation of urea to ammonia and carbon dioxide to eliminate urea from the waste water stream formed during urea production. These processes involve introducing the waste water stream into a carbon dioxide recovery system, optionally in the presence of vanadium pentoxide. These patents, though, do not suggest the use of the urea hydrolyzation products for nitrogen oxides reduction, and especially not the use of hydrolyzation products ammonium carbamate, ammonium carbonate, ammonium bicarbonate, and ammonia for NO.sub.x reduction without generating N.sub.2 O.
What is desired, therefore, is a system whereby nitrogen oxides reductions can be achieved while taking advantage of the advantages of the use of urea without the concommitant risk of generation of substantial amounts of N.sub.2 O.